(a) Field of the Invention
The present invention relates to high strength PAN fiber composed of a high molecular weight AN polymer and a method of producing the same.
(b) Description of the Prior Art
PAN fiber, one of the "three big fibers" and ranking with nylon and polyester fibers, is widely used in the field of wearing apparel which makes the most of its characteristics such as clearness of dyed color, bulkiness, etc. The strength of PAN fiber for use in such wearing apparel is in the order of 3 to 4 g/d.
Carbon fiber produced by carbonizing PAN fiber is used as a reinforcing fiber for composite materials because of its excellent physical properties (high strength, high modulus of elasticity). Since the surface condition, cross-sectional shape, physical properties, etc. of the carbon fiber are determined for the most part by the characteristics of the starting material PAN fiber (precursor), its improvements are contemplated actively. However, the strength of the precursor produced on an industrial scale is generally limited to about 5 to 8 g/d.
On the other hand, the aromatic polyamide fibers represented by Kevlar.RTM. produced by DuPont, have a strength higher than 20 g/d owing to their stiff molecular structure, and therefore they are establishing a firm position as reinforcing fiber for tire cord and composite material.
In such a situation, appearance of a high strength PAN fiber is expected that can be used as precursor of highly reliable carbon fiber serviceable for astronautics and aeronautics, or that can be used as reinforcing fiber singly. As an attempt in this regard, Japanese Pat. No. 52125/1981 describes that a high strength PAN fiber can be produced by a special technique which comprises solution-polymerizing AN in a concentrated solution of a complex salt (NaZnCl.sub.3), under the action of ultra violet rays, in the presence of formaldehyde and hydrogen peroxide; spinning the thus-obtained solution directly into a coagulation bath; and stretching the resulting fibers at the time of coagulation, thereby to form an oriented tissue in the skin portion. However, even by this method, a strength of 16 g/d is attained at the highest.